1. Field of the Invention
This invention relates to beam shaping, and, more particularly, to beam stacking devices and methods for stacking beams which change the shape of a beam from a highly asymmetric non-diffraction-limited laser source, such as a high power laser diode bar, to a more symmetric, and thus more desirable and useful, profile.
2. Description of the Prior Art
While semiconductor lasers are widely used in both scientific and consumer devices, the relatively low power output of such devices has limited their use. The output power of semiconductor lasers has been increased by several different techniques, one being fabricating a multiple diode device in the form of an array of low power laser diodes on a single chip. While it is possible to fabricate such a multiple diode device having, for example, 10 laser diodes, the output of such a device is a beam having a highly elliptical shape or profile. That is, the output beam is much wider in the dimension corresponding to the necessary spacing between the laser diodes than in the dimension corresponding to the width of a single laser diode. In other words, the output beam has dramatically unequal M.sup.2 dimensions in two orthogonal directions, thus resulting in a beam which is highly asymmetrical.
High power laser diode bars are attractive as optical sources for applications in pumping solid state lasers, in materials processing, and in the medical field. Unfortunately, the inconvenient shape, i.e., highly elliptical nature, of the output beam of laser diode bars makes them, by themselves, inappropriate for many of the uses which require a higher power output than is attainable from a single laser diode source. However, this has not been a fatal shortcoming because there exist techniques for reconfiguring the shape or profile of the output beam to make it less elliptical in nature; that is, to create an output beam whose dimensions in two orthogonal directions are more nearly equal, thus giving rise to an output beam which is substantially symmetrical. One such technique is the beam stacking device disclosed in European patent number EP0731932 and in corresponding International Application (PCT) number WO 95/15510, and it is that sort of technique which the present invention serves to improve.
The beam stacking device of European patent number EP0731932 and International Application number WO 95/15510 consists of two high reflectivity planar mirrors aligned approximately parallel and separated by a small distance. The two mirrors are offset transversely from each other in two directions so that small sections of each mirror are not obscured by the other. These unobscured sections form input and output apertures of the beam stacking device. The action of the beam stacking device is to chop the incident beam into a specific number of chopped parts and then to redirect and reposition these chopped parts so that they emerge from the beam stacking device one on top of another. Although this beam stacking device does reconfigure the output beam to a more favorable profile, it nevertheless has several disadvantages, the principal disadvantage being that, because the incident beam is chopped many times and many reflections are needed between the mirrors, losses due to reflection are great and, therefore, the transmission is low.